Over the past four decades, intracellular recordings from sensory cortex of anesthetized animals have made major contributions to our understanding of cortical processing, providing a window into the synaptic inputs that shape spiking responses of individual cortical neurons. To date, however, intracellular recording has not been applied in awake, behaving primates. By combining the unique expertise from our two laboratories, we have recently developed novel techniques that allow us to conduct, on a routine basis, reliable, whole-cell intracellular recordings in primary visual cortex (V1) of awake, behaving monkeys. For the first time, we have access to both subthreshold (membrane potentials representing input) and suprathreshold (spikes representing output) responses of individual cortical neurons, while also utilizing the precise control of visual stimulation and the subject's behavioral state afforded by behaving primates. Our ability to perform intracellular recordings in awake, behaving primates opens the door to addressing two fundamental questions with respect to the circuit-level mechanisms that mediate visual perception: (1) what is the relationship between sub- and supra-threshold activity of single cortical neurons and perceptual decisions, and (2) what are the underlying mechanisms of top-down attentional modulations in sensory cortex. To address these questions, we will first study the quantitative relationship between variability in sub- and suprathreshold responses of single V1 neurons and variability in perceptual decisions of monkeys performing a demanding visual detection task (Aim 1). In Aim 2, we will examine how sub- and suprathreshold responses are altered by changing the attentional state under which the stimulus is presented. We present preliminary data demonstrating that these recordings are not only technically feasible, but are also able to provide important and unique insights into the cellular and circuit-level mechanisms that mediate cortical sensory processing.